Do we need to master Cycles' material nodes if we can make textures/materials in a much easier way in Substance Painter?
There is no way to answer this question with a 'yes' based on facts. You do not need to master anything, but you might want to learn to use the nodes in order to make your life easier. Cycles nodes are for material creation in Cycles so no matter how you make your textures, if you use Cycles you might want to learn its functionality. Nodes can be useful for many reasons not related to the look of a material directly as well - you can manipulate texture coordinates for example or randomize material attributes or control them based on objects position or surface direction and so on. It's just useful, not so much necessary if you only use Principled BSDF shaders.

My advice(opinion): The word 'master' sounds intimidating, but in reality, you just need to learn what the nodes do. Just go through the add menu and figure out what each one does and think of how you can use them and that will get you started. There are like 80 or so of them, but they are in categories so it's not that hard. If you decide to go on an see a few tutorials on how to make some procedural textures and learn how to manipulate coordinates with math you are already an 'advanced' user.